1. Field of the Invention
The present invention relates to a separator for a closed alkaline secondary battery such as a nickel-hydrogen battery or a nickel-cadmium battery and a method for producing such a separator.
More specifically, the present invention relates to a secondary battery separator having an excellent function of trapping impurities in an alkaline electrolytic solution, a battery using such a separator, and a method for producing the inventive secondary battery by giving a hydrophilic functional group to a material for the separator.
2. Description of the Prior Art
As shown in FIG. 5, a positive electrode and a negative electrode are separated by a separator and accommodated in a container while being immersed in an electrolytic solution in a closed alkaline secondary battery. A self-discharging phenomenon that a charged amount decreases with time if a charged alkaline secondary battery is kept is known. Particularly, a nickel-hydrogen battery using a hydrogen-occluded alloy as a negative electrode is known to have a considerable reduction in the charged amount due to self-discharging. For example, a separator using polyamide fibers is hydrolyzed by an electrolytic solution to produce nitride compounds. Such nitride compounds cause the self-discharging phenomenon. Substances that cause the self-discharging phenomenon such as nitrate groups are produced not only by hydrolysis of the separator, but also by oxidation of ammonia mixed in the battery. Accordingly, an amount of nitrate groups can be reduced by reducing an amount of ammonia present in the secondary battery, thereby enabling suppression of self-discharging. However, since ammonia is likely to be mixed, as impurities, in a manufacturing process of electrodes, it is difficult to completely prevent ammonia from being mixed into the battery.
Accordingly, there has been proposed a separator which is not hydrolyzed in an electrolytic solution unlike the separator made of polyamide fibers and can trap ammonia in the electrolytic solution. For example, there has been proposed a technique of introducing functional groups having an ion exchange ability such as sulfonic groups and carbonate groups into polyolefin fibers by applying various hydrophilic treatments such as sulfonation and graft polymerization of hydrophilic monomers to the polyolefin fibers.
Separators having a high ion exchange amount which are obtained by applying a hydrophilic treatment to polyolefin fibers has been proposed (Japanese Unexamined Patent Publications No. 10(HEI)-326607, No. 10(HEI)-116600). However, the separator into which functional groups are introduced by applying a hydrophilic treatment by graft polymerization lacks practicability since it has a low heat resistance although having a high self-discharging suppressing performance at room temperature. For example, if this separator is used in an environment of a relatively high temperature of 60° C. or higher, carboxylic acid groups fall off from the separator, resulting in a reduced ammonia trapping performance and according an increased self-discharging amount.
There is a technique of applying sulfonation to polyolefin fibers to obtain a separator in which functional groups are stable at relatively high temperature. However, polyolefin fibers such as polyethylene fibers and polypropylene fibers are difficult to sulfonate since they have a high acid resistance. Accordingly, if sulfonic groups are introduced to the insides of the fibers, it causes a problem of reduced strength of the fibers.
There is also known a technique of applying sulfonation to fibers having a relatively low acid resistance such as polystyrene fibers. Sulfonated polystyrene fibers are poor in ammonia trapping performance and have a considerable progress of self-discharging since sulfonic groups can be easily introduced into the outer surfaces of the fibers, although they have a high ion exchange amount.
The separators produced using the fibers to which various treatments such as sulfonation are applied have all a high capacity retaining rate and a suppressed degree of self-discharging as compared to the conventional separators. However, these separators also have a problem of reduced strength and are by no means said to possess a sufficient practicability.
Further, in response to a demand for higher capacity and higher output of a battery, a technique of reducing a occupied volume of the separator in a battery by making the thickness of the separator maximally thinner has been developed in recent years. For example, in order to increase a capacity of a nickel-hydrogen battery, thin separators having a thickness of 150 μm or smaller, or even 120 μm or smaller have been developed. However, in order to put secondary batteries having a high capacity to practical use, there has been a demand for separators which can attain a high capacity retaining rate by suppressing self-discharging, and have sufficient strength.
Various types of thin separators have been proposed thus far. These proposed separators include, for example, one made thinner by compressing a separator, into which functional groups are introduced by sulfonation, at high pressure. However, oxygen gas permeability is reduced as the separator is compressed, and an inner pressure is likely to rise due to oxygen gas produced at a final stage of charging. Thus, there are dangers of a liquid leak or destruction of the battery.
In order to thin the separator without causing a liquid leak and destruction of the battery as the oxygen gas permeability of the separator decreases, a technique of decreasing a Metsuke (g/m2) of the separator has been proposed. However, this technique has a problem that the strength of the separator decreases as the Metsuke decreases. Particularly, since the piercing strength of the separator is considerably reduced as the Metsuke decreases, flushes of the electrodes pierce the separator during a manufacturing process of the battery, thereby causing a short circuit. This results in an increase in the defect rate of the battery.
A method for increasing a fiber density is being considered as a technique of suppressing an occurrence of a short circuit resulting from the reduced Metsuke. However, since a spatial volume within the fibers decreases if the fiber density is increased, liquid retaining property and the gas permeability of the separator decrease. This causes a problem of a large increase in the inner pressure of the battery.
With the prior art technique, it has been difficult to maintain the strength of the separator and to improve a capacity retaining rate of the separator.